Method of calibrating a constant voltage supply for an ultrasonic transducer of a wire bonding machine

ABSTRACT

A method of calibrating a voltage for use with an ultrasonic transducer of a wire bonding machine in a constant voltage mode is provided. The method includes: (1) determining an impedance value associated with operation of the ultrasonic transducer; and (2) establishing an adjusted voltage level for use with the ultrasonic transducer in the constant voltage mode based on the determined impedance value.

CROSS REFERENCE

This application claims the benefit of International Application No.PCT/US2007/088482 filed Dec. 21, 2007, the contents of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the application of energy to anultrasonic transducer of a wire bonding machine, and more particularly,to improved methods of applying a constant voltage to an ultrasonictransducer.

BACKGROUND OF THE INVENTION

In the processing and packaging of semiconductor devices, wire bondingcontinues to be the primary method of providing electricalinterconnection between two locations within a package (e.g., between adie pad of a semiconductor die and a lead of a leadframe). Morespecifically, using a wire bonder (also known as a wire bonding machine)wire loops are formed between respective locations to be electricallyinterconnected.

An exemplary conventional wire bonding sequence includes: (1) forming afree air ball on an end of a wire extending from a bonding tool; (2)forming a first bond on a die pad of a semiconductor die using the freeair ball; (3) extending a length of wire in a desired shape between thedie pad and a lead of a leadframe; (4) stitch bonding the wire to thelead of the leadframe; and (5) severing the wire. In forming the bondsbetween (a) the ends of the wire loop and (b) the bond site (e.g., a diepad, a lead, etc.) varying types of bonding energy may be usedincluding, for example, ultrasonic energy, thermosonic energy,thermocompressive energy, amongst others.

As is known to those skilled in the art, these energy sources are notapplied in a mutually exclusive way. For example, thermosonic energytypically involves the application of heat (e.g., from a heat block) andultrasonic energy (e.g., from an ultrasonic transducer). When usingultrasonic energy in connection with wire bonding, there are generallytwo forms of ultrasonic output control: constant current control mode,where the current applied to the transducer is held constant (or is heldto a predefined current profile) while the voltage may be varied; andconstant voltage control mode, where the voltage applied to thetransducer is held constant (or is held to a predefined voltage profile)while the current may be varied. In some applications, a constant powermode has also been used.

Many early wire bonder platforms used constant voltage mode in open loopcontrol of current (i.e., the voltage applied is at a constant levelregardless of the impedance variation of the system). Later wire bonderplatforms adopted constant current control mode which allowed for moreof a closed loop control. That is, the current may be fed back to acontrol board, whereby the voltage is adjusted to keep the desiredcurrent.

As is known to those skilled in the art, a benefit of constant currentcontrol mode is that it enables “portability” between one wire bondingsystem/machine and another. That is, the displacement of the transducer(and hence the capillary) is proportional to the current of the system.Therefore, in order to achieve similar bonding results (e.g., balldiameter, ball shear, etc.) for systems with different impedance values,supplying the same current to the transducer often yields acceptableresults. One drawback of the constant current mode is that when aresonant frequency of the transducer is close a resonant frequency ofthe bonding components (e.g., a first bond die pad, a second bond leadof a leadframe), the impedance of the system may change significantly(e.g., the impedance may increase significantly). In a constant currentcontrol mode the system will attempt to adjust to this change inimpedance by changing the voltage output (e.g., the voltage willsignificantly increase to account for a significant increase inimpedance). For example, this may result in an increase to the overallenergy put into the bond, which in turn may cause inconsistency inbonding (e.g., over squashed bonds, second bond short tailinconsistency, etc.).

In contrast, constant voltage control mode desirably limits the energyoutput to the bond when there is a resonance problem, and as such, thebonding results tend to be much more consistent. Unfortunately, a drawback of constant voltage control mode is a general lack of portability.Since the impedance of the systems are not the same (e.g., due to themechanical differences of the transducer, coupling differences betweenthe transducer and the mounting structure, mounting differences betweenthe transducer and the capillary, etc.), the impedance from system tosystem may vary considerably (e.g., one system impedance may be 20 ohms,while another system impedance may be 50 ohms). Thus, when using theconstant voltage control mode, the system with a lower impedanceundesirably results in more energy being applied to the bonds than ahigher impedance system.

As such, there are clear limitations in both the conventional constantcurrent and constant voltage control modes. Thus, it would be desirableto provide improved methods of applying a constant voltage to anultrasonic transducer of a wire bonding machine.

SUMMARY OF THE INVENTION

According to an exemplary embodiment of the present invention, a methodof calibrating a voltage for use with an ultrasonic transducer of a wirebonding machine in a constant voltage mode is provided. The methodincludes: (1) determining an impedance value associated with operationof the ultrasonic transducer; and (2) establishing an adjusted voltagelevel for use with the ultrasonic transducer in the constant voltagemode based on the determined impedance value.

The methods of the present invention may also be embodied as anapparatus (e.g., as part of the intelligence of a wire bonding machine),or as computer program instructions on a computer readable carrier(e.g., a computer readable carrier used in connection with a wirebonding machine).

According to another exemplary embodiment of the present invention, awire bonding system is provided. The wire bonding system includes anultrasonic transducer and a controller. The controller is configured to(1) determine an impedance value associated with operation of theultrasonic transducer, and (2) establish an adjusted voltage level foruse with the ultrasonic transducer in a constant voltage mode based onthe determined impedance value.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1 is a perspective view of a transducer of a wire bonding machineand related components in accordance with an exemplary embodiment of thepresent invention;

FIG. 2 is a block circuit diagram useful in understanding methods ofdetermining an impedance value associated with operation of anultrasonic transducer in accordance with an exemplary embodiment of thepresent invention;

FIG. 3 is a block circuit diagram useful in understanding methods ofcalibrating a voltage for use with an ultrasonic transducer of a wirebonding machine in a constant voltage mode in accordance with anexemplary embodiment of the present invention;

FIG. 4 is a flow diagram illustrating a method of calibrating a voltagefor use with an ultrasonic transducer of a wire bonding machine in aconstant voltage mode in accordance with an exemplary embodiment of thepresent invention;

FIG. 5 is a flow diagram illustrating another method of calibrating avoltage for use with an ultrasonic transducer of a wire bonding machinein a constant voltage mode in accordance with an exemplary embodiment ofthe present invention; and

FIG. 6 is a flow diagram illustrating a method of determining animpedance value associated with operation of the ultrasonic transducerin accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with certain exemplary embodiments of the presentinvention, a constant voltage control mode is used to provide energy toan ultrasonic transducer of a wire bonding system. The constant voltageis preferred in certain applications, for example, because constantvoltage mode tends to be less sensitive to poor lead finger clamping andlead finger resonance conditions, amongst other potential issues. Inorder to overcome portability issues with conventional constant voltagecontrol modes, the impedance of the ultrasonic transducer (and/or theimpedance associated with the operation of the ultrasonic transducer)may be used to normalize the amplitude output of the voltage.

By using the improved constant voltage mode described herein, a numberof advantages are achieved including, for example, improved second bondon applications with lead fingers that are subject to resonance whilestill maintaining machine to machine portability and compensating forchanges in system impedance due to wear and time effects.

As used herein, the term “controller” is intended to be broadly definedas a portion of a wire bonding system/machine including one or more of anumber of functions, such as: signal generation for transmission to theultrasonic transducer, signal amplification of the generated signal,machine software functionality, control board components, logic, etc.Thus, it is clear that the term controller is not intended to be limitedto any specific component of a wire bonding system.

In the present application, the impedance associated with operation ofan ultrasonic transducer is described. As is known to those skilled inthe art, the impedance associated with the operation of an ultrasonictransducer is an impedance value at a frequency at which the transducermay operate. This is typically at or near a resonant frequency of theultrasonic transducer (as is known to those skilled in the art, anultrasonic transducer may have multiple resonant frequencies). Theimpedance associated with the operation of the ultrasonic transducer mayinvolve impedance components from the transducer itself, impedance fromthe mounting of the transducer, etc.

FIG. 1 is a perspective view of ultrasonic transducer 100. Transducer100 includes mounting flanges 102 a and 102 b used to mount transducer100 to a bond head of a wire bonding machine. Piezoelectric crystals 104are provided in a cavity defined by transducer 100, where crystals 104provide ultrasonic energy to transducer 100 via the application ofelectrical energy to electrodes 106 a and 106 b. This ultrasonic energyis transmitted to bonding tool 108 which is used in wire bonding, studbumping and the like.

FIG. 2 is a block circuit diagram useful in understanding methods ofdetermining an impedance value associated with operation of anultrasonic transducer in accordance with an exemplary embodiment of thepresent invention. More specifically, in order to find a resonantfrequency of an ultrasonic system (and therefore, to determine animpedance value associated with operation of the ultrasonic system),controller 204 includes signal generator 200 and amplifier 202. Signalgenerator 200 produces a sinusoidal output signal which is swept over awide frequency range including a resonant frequency of the ultrasonictransducer. This sinusoidal output signal is amplified by amplifier 202,and then this amplified signal is sent to transducer 206. A feedbacksignal (e.g., a feedback signal related to the voltage and current ofthe signal running through the transducer) is sent to controller 204.Controller 204 maintains a constant voltage during the frequency sweep.During this process, controller 204 monitors the feedback signal todetermine when the frequency of the signal approaches a resonantfrequency of the ultrasonic system. Of course, there are numerousmethods by which a resonant frequency may be sensed.

In a first example, the feedback signal may be monitored to sense anincrease in current. When the current in the system reaches a maximum(i.e., of course, there may be multiple current peaks or “maximums” fora given ultrasonic system, but there may be only one targeted currentmaximum in the frequency sweep of the signal), the controller determinesthat a resonant frequency has been reached. At this frequency, thevoltage and current are sampled and used to determine the ultrasonicsystem impedance at the resonant frequency.

In a second example, the feedback signal may be monitored to sense thephase difference between the voltage and current. When the phasedifference is approximately zero (i.e., the voltage and current are “inphase”), the controller determines that a resonant frequency has beenreached. At this frequency, the voltage and current are sampled and usedto determine the ultrasonic system impedance at the resonant frequency.

Regardless of the technique utilized, the resonant frequency is sensedby the controller, and then the impedance associated with the operationof the ultrasonic transducer is determined. After the ultrasonic systemimpedance is determined, it may be used to adjust the voltage that willbe applied to the ultrasonic transducer in a constant voltage controlmode. For example, FIG. 3 is a block circuit diagram useful inunderstanding methods of calibrating a voltage for use with anultrasonic transducer of a wire bonding machine in a constant voltagemode in accordance with an exemplary embodiment of the presentinvention.

Referring now to FIG. 3, controller 300 includes various elements suchas machine software 302 and ultrasonic control board/generator 304.Machine software 302 has access to a reference impedance (Z_(ref)) and areference voltage (V_(ref)). For example, the reference impedance andreference voltage may represent values for a known/representativesystem. Thus, the reference impedance is a predetermined value which isan impedance value associated with operation of a known/representativeultrasonic transducer. The reference voltage is a voltage value that hasbeen used in conjunction with a constant voltage mode of theknown/representative ultrasonic transducer. For example, the referenceimpedance and the reference voltage values may be stored in memory of awire bonding machine, or may be entered into the wire bonding machine(e.g., by a user entering the values using a graphical user interface orthe like).

Machine software 302 also has access to the actual impedance value(Z_(actual)) associated with operation of the subject ultrasonictransducer. For example, the actual impedance value (Z_(actual)) may bedetermined as described above with respect to FIG. 2.

Machine software 302 establishes/calculates a ratio of the actualimpedance to the reference impedance value (Z_(actual)/Z_(ref)), and themachine software subsequently applies this ratio to the referencevoltage to provide an adjusted voltage level. This adjusted voltagelevel may be the adjusted voltage level used by ultrasonic controlboard/generator 304; however, as shown in FIG. 3, a portability factormay also be applied to the adjusted voltage level to reach a furtheradjusted level. For example, such a portability factor may be determinedbased on empirical testing/data related to the subject transducer, wherethe portability factor is related to the gain/sensitivity of theultrasonic system. That is, if a certain input is applied to theultrasonic transducer, is the output the desired output? Or is there aloss, such as an ultrasonic crystal loss, which makes the use of aportability factor desirable? An example illustrates the function of theblock diagram of FIG. 3 quite well.

In this example, the goal is to determine the voltage to be applied to asubject ultrasonic transducer in a constant voltage mode. This may be aone-time determination (e.g., at the start-up of the wire bondingmachine) or this may be a repetitive or cyclical determination. Assumethat previous testing of a reference ultrasonic system at a resonantfrequency of 120 KHz yields reference impedance and reference voltagevalues as follows: Z_(ref)=30 ohms; and V_(ref)=10 volts. Through anoperation such as that described above with respect to FIG. 2, an actualimpedance associated with operation of the ultrasonic transducer isdetermined to be Z_(actual)=40 ohms. Machine software 302 determinesthat the ratio is 1.33 (i.e., Z_(actual)/Z_(ref)=40/30=1.33). Thus, thisratio is applied V_(ref), whereby an adjusted voltage value isdetermined to be 13.3 volts (i.e., Adjusted voltage=(V_(ref)*Ratio)=(10volts*1.33)=13.3 volts). This adjusted voltage may be communicated toultrasonic control board/generator 304, through which a constant voltageof 13.3 volts is applied to the subject ultrasonic transducer.

Alternatively, a portability factor (PF) may be applied to the adjustedvoltage. For example, using empirical testing, it may be determined thesubject ultrasonic system has a certain loss by which it is desirable toprovide a portability factor of 1.05. Thus, the adjusted voltage (13.3volts) is multiplied by the portability factor (1.05) to provide afurther adjusted voltage of 13.965 volts. This adjusted voltage may becommunicated to ultrasonic control board/generator 304, through which aconstant voltage of 13.965 volts is applied to the subject ultrasonictransducer.

FIGS. 4-6 are flow diagrams in accordance with certain exemplaryembodiments of the present invention. As is understood by those skilledin the art, certain steps included in the flow diagrams may be omitted;certain additional steps may be added; and the order of the steps may bealtered from the order illustrated.

FIG. 4 is a flow diagram illustrating a method of calibrating a voltagefor use with an ultrasonic transducer of a wire bonding machine in aconstant voltage mode in accordance with an exemplary embodiment of thepresent invention. At step 400, an impedance value associated withoperation of the ultrasonic transducer is determined. For example, thisimpedance value may not only be the impedance of the ultrasonictransducer, but may also include impedance components related to, forexample: coupling of the piezoelectric crystals within the transducer;the electrical connections to the transducer; the mounting of thetransducer; the coupling between the bonding tool and the transducer,etc. At step 402, an adjusted voltage level is established for use withthe ultrasonic transducer in the constant voltage mode based on thedetermined impedance value. For example, as described above, a ratio ofthe actual impedance to a reference impedance may be calculated, andthis ratio may be applied to a reference voltage to reach the adjustedvoltage. Of course, alternative techniques of adjusting the voltagebased on the determined impedance value are contemplated. This adjustedvoltage may be the voltage applied to the transducer. In thealternative, at optional step 404, a further adjusted voltage value maybe calculated, where the further adjusted voltage is the adjustedvoltage multiplied by a portability factor.

FIG. 5 is a flow diagram illustrating another method of calibrating avoltage for use with an ultrasonic transducer of a wire bonding machinein a constant voltage mode in accordance with an exemplary embodiment ofthe present invention. At step 500, a reference voltage (V_(ref)) and areference impedance (Z_(ref)) are established. As provided above, thereference impedance and reference voltage may represent values for aknown/representative system. At step 502, an impedance value associatedwith operation of the ultrasonic transducer is determined (Z_(actual)).At step 504, a ratio of the impedance determined in step 502 to thereference impedance is established (i.e., Z_(actual)/Z_(ref)). At step506, an adjusted voltage level is established for use with theultrasonic transducer in the constant voltage mode by applying the ratioto the reference voltage. This adjusted voltage may be the voltageapplied to the transducer. In the alternative, at optional step 508, afurther adjusted voltage value may be calculated, where the furtheradjusted voltage is the adjusted voltage multiplied by a portabilityfactor.

FIG. 6 is a flow diagram illustrating a method of determining animpedance value associated with operation of the ultrasonic transducerin accordance with an exemplary embodiment of the present invention. Forexample, the method shown in FIG. 6 may be used in connection with Step400 of FIG. 4, or in connection with step 502 of FIG. 5.

Referring again to FIG. 6, at step 600 a sinusoidal output signal isproduced using a controller of a wire bonding machine. The sinusoidaloutput signal is swept across a frequency range including a resonantfrequency of the ultrasonic transducer. At step 602, the sinusoidaloutput signal is amplified (e.g., using an amplifier of the controller).At step 604, the amplified sinusoidal output signal is sent to thetransducer. At step 606, a feedback signal of the amplified sinusoidaloutput signal is measured using the controller. At step 608, a resonantfrequency of the ultrasonic transducer is determined through themeasurement of the amplified sinusoidal output signal in step 606. Forexample, this resonant frequency may be determined by any of a number oftechniques (e.g., monitoring the signal current for a current maximum,monitoring the phase difference between the voltage and current of thesignal, etc.). At step 610, the impedance value associated withoperation of the ultrasonic transducer is determined at the resonantfrequency.

The calibration techniques disclosed herein (e.g., the calibration of aconstant voltage for use with an ultrasonic transducer) may be performedone time for a given wire bonding machine/system. Alternatively, thecalibration techniques may be performed at a given frequency (e.g., atevery bond, at every 100 bonds, etc.) or at a given event (e.g., eachtime a new capillary tool is used, etc.).

Although certain aspects of the present invention are described inconnection with a resonant frequency of an ultrasonic tranducer, it isknown to those skilled in the art that an ultrasonic transducer may havemultiple resonant frequencies. As such, these aspects of the presentinvention may be utilized in connection with any of the resonantfrequencies.

Certain exemplary embodiments of the present invention described hereinrelate to the establishment of an adjusted voltage level for use withthe ultrasonic transducer in the constant voltage mode based on animpedance value determined for the subject transducer. In one example,the voltage level is adjusted (based on the impedance value) through theapplication of a ratio (Z_(actual)/Z_(ref)) to a reference voltage. Ofcourse, the voltage level may be adjusted (based on the impedance level)in different ways within the scope of the present invention. Forexample, the voltage level may be adjusted using a different function orratio that is based on the impedance value of the subject transducer.Other techniques are also contemplated.

The calibration techniques of the present invention may be implementedin a number of alternative mediums. For example, the techniques can beinstalled on an existing computer system/server as software (a computersystem used in connection with, or integrated with, a wire bondingmachine). Further, the techniques may operate from a computer readablecarrier (e.g., solid state memory, optical disc, magnetic disc, radiofrequency carrier medium, audio frequency carrier medium, etc.) thatincludes computer instructions (e.g., computer program instructions)related to the calibration techniques.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

What is claimed:
 1. A method of calibrating a voltage for use with anultrasonic transducer of a wire bonding machine in a constant voltagemode, the method comprising the steps of: (1) determining an impedancevalue associated with operation of the ultrasonic transducer; and (2)establishing an adjusted voltage level for use with the ultrasonictransducer in the constant voltage mode based on the determinedimpedance value.
 2. The method of claim 1 further comprising a step ofestablishing a reference voltage and a reference impedance value, andestablishing a ratio of the impedance value determined in step (1) tothe reference impedance value (Z_(actual)/Z_(ref)), whereby the adjustedvoltage level is established in step (2) by applying the ratio to thereference voltage.
 3. The method of claim 1 wherein step (1) includesmeasuring a current signal passing through the ultrasonic transducersuch that a maximum current value may be detected, and then determiningthe impedance value at the maximum current value.
 4. The method of claim1 wherein step (1) includes measuring a phase difference between thecurrent and voltage of a signal passing through the ultrasonictransducer such that when the phase difference is substantially zero theimpedance value may be determined at step (1).
 5. The method of claim 1wherein step (1) includes (a) producing a sinusoidal output signal usinga controller of the wire bonding machine, where the sinusoidal signal isswept across a frequency range including a resonant frequency of theultrasonic transducer; (b) amplifying the sinusoidal output signal; (c)sending the amplified sinusoidal output signal to the transducer; (d)measuring, using the controller, a feedback signal of amplifiedsinusoidal output signal sent to the transducer; (e) determining theresonant frequency of the ultrasonic transducer through step (d); and(f) determining the impedance value at the resonant frequency of theultrasonic transducer.
 6. The method of claim 5 wherein step (e)includes measuring a current value of the feedback signal, anddetermining the resonant frequency by determining a maximum currentvalue of the feedback signal.
 7. The method of claim 5 wherein step (e)includes measuring a phase difference between a current and a voltage ofthe feedback signal, and determining the resonant frequency when thephase difference is substantially zero.
 8. The method of claim 1 whereinstep (1) includes (a) determining a resonant frequency of the ultrasonictransducer; and (b) determining the impedance value at the resonantfrequency of the ultrasonic transducer.
 9. The method of claim 1 furthercomprising the step of: (3) further adjusting the adjusted voltage levelusing a portability factor determined in connection with operation ofthe ultrasonic transducer.
 10. A computer readable carrier includingcomputer program instructions which cause a computer to implement amethod of calibrating a voltage for use with an ultrasonic transducer ofa wire bonding machine in a constant voltage mode, the method comprisingthe steps of: (1) determining an impedance value associated withoperation of the ultrasonic transducer; and (2) establishing an adjustedvoltage level for use with the ultrasonic transducer in the constantvoltage mode based on the determined impedance value.
 11. A wire bondingsystem comprising: an ultrasonic transducer; and a controller, thecontroller being configured to (1) determine an impedance valueassociated with operation of the ultrasonic transducer, and (2)establish an adjusted voltage level for use with the ultrasonictransducer in a constant voltage mode based on the determined impedancevalue.
 12. The wire bonding system of claim 11 wherein the wire bondingsystem includes a reference voltage value and a reference impedancevalue stored in memory, and wherein the controller is configured toestablish a ratio of the impedance value to the reference impedancevalue (Z_(measured)/Z_(reference)), and wherein the controller isconfigured to establish the adjusted voltage level by applying the ratioto the reference voltage.
 13. The wire bonding system of claim 11wherein the controller is configured to measure a current signal passingthrough the ultrasonic transducer such that a maximum current value maybe detected, and then the controller determines the impedance value atthe maximum current value.
 14. The wire bonding system of claim 11wherein during the determination of the impedance value associated withoperation of the ultrasonic transducer, the controller is configured tomeasure a phase difference between a current and a voltage of a signalpassing through the ultrasonic transducer such that when the phasedifference is substantially zero the impedance value is determined. 15.The wire bonding system of claim 11 wherein during the determination ofthe impedance value associated with operation of the ultrasonictransducer, the controller is configured to (a) produce a sinusoidaloutput signal swept across a frequency range including a resonantfrequency of the ultrasonic transducer; (b) amplify the sinusoidaloutput signal; (c) send the amplified sinusoidal output signal to thetransducer; (d) measure a feedback signal of amplified sinusoidal outputsignal sent to the transducer; (e) determine the resonant frequency ofthe ultrasonic transducer; and (f) determine the impedance value at theresonant frequency of the ultrasonic transducer.
 16. The wire bondingsystem of claim 15 wherein during step (e), the controller is configuredto measure a current value of the feedback signal, and determine theresonant frequency by determining the maximum current value of thefeedback signal.
 17. The wire bonding system of claim 15 wherein duringstep (e), the controller is configured to measure a phase differencebetween the current and voltage of the feedback signal, and to determinethe resonant frequency when the phase difference is substantially zero.18. The wire bonding system of claim 11 wherein during the determinationof the impedance value associated with operation of the ultrasonictransducer, the controller is configured (a) determine a resonantfrequency of the ultrasonic transducer; and (b) determine the impedancevalue at the resonant frequency of the ultrasonic transducer.
 19. Thewire bonding system of claim 11 wherein the controller is configured tofurther adjust the adjusted voltage level using a portability factordetermined in connection with operation of the ultrasonic transducer.